This invention relates to a printed circuit board and method of preparing such a board and more particularly to an improved and simplified method of making a printed circuit on a film type insulator.
The use of printed circuit boards is well established and well known. Such circuit boards are formed by applying, in some manner, discrete circuitry on an insulating board. Such arrangements have high utility and are used in a wide variety of applications. Although great progress has been made with circuit boards, still further improvement is desirable.
For example, it is well known that a wide number of circuits can be formed on a single board. However, with the present trend toward further miniaturization and with increasing complexity of the circuits, it is desirable to provide a method for forming a printed circuit wherein an even greater number of circuit components may be formed on a given circuit board. Furthermore, in many applications it is desirable to form the circuit on a board that not only is relatively thin but one which may also be flexible. Such film type printed circuit boards have particular utility in electric circuits for cameras, copy machines and the like.
One method in which a printed circuit of this general type has been formed would be to coat a metal layer such as aluminum, copper or the like on the entire surface of a substrate by a PVD method such as vacuum deposition or sputtering or a CVD method. The upper surface of the metal coating is then provided with a photoresistant layer of a given pattern to form the desired circuits. Thereafter, the unprotected portions of the metal layer are removed by etching to form the wiring pattern on the substrate. These methods require a high vacuum technique and manufacturing at a high temperature. Therefore, the type of substrate used is quite limited and the substrates must have a fair degree of thickness. Therefore, this method cannot be used practically on a thin membrane substrate such as a film.
There is another form of process which does not require high vacuum techniques and which can be used to form a wiring pattern on a thin membrane such as a film. In this method, a metal base is electroplated or chemically plated in a given pattern to form the desired circuits. An insulated film is then bonded to this pattern and the film is peeled from the substrate to transfer the wiring pattern to the film. The disadvantage with this type of system is that the peeling of the film can put undue stresses on the discrete circuitry and cause breaks or high resistance in the circuits.
It is, therefore, a principal object of this invention to provide an improved method for forming a printed circuit that lends itself to formation of printed circuits on thin films.
It is a further object of this invention to provide an improved and simplified method for forming printed circuits on a board wherein the board and circuit will be strong and interruptions in the circuitry will not result during the forming process.
It is a further object of this invention to provide an improved and simplified method for forming printed circuits on thin substrates such as films and wherein relatively simple manufacturing techniques can be employed.
In many forms of printed circuits, the substrate on which the circuit has been formed is further protected by bonding an insulating layer to the printed circuit board so that other circuits can be formed on top of the first formed circuit. However, coating of the printed circuit board does not necessarily provide a generally flat surface. Normally, the way coating is accomplished is that the printed circuit board is enveloped within a film. However, the circuitry extends above the circuit board and when the film is put in place, it does not have a perfectly planar surface. In addition, this method of coating or insulating the printed circuit lends itself to the entrapment of air which can cause degradation of the printed circuit.
It is, therefore, a further object of this invention to provide an improved method for applying a protective coating to a printed circuit.
It is another object of this invention to provide a method for coating a printed circuit in such a way that a smooth and perfectly planar layer will result so as to permit stacking of circuits one upon the other.
It is a further object of this invention to provide an improved method for coating a printed circuit board while avoiding the intrusion of air into the coating.
As has been previously noted, one method by which printed circuits can be formed, particularly on film layers, is by depositing a metal on the film layer and then etching the metal to remove unwanted portions to result in the desired circuit formation. Another problem with this etching technique is that the side edges of the printed circuit tend to become tapered being wider at the base where they are attached to the film than at the top. As a result of this decreased cross-sectional area, the circuit components must be wider so as to avoid high resistivity. As a result, the number of circuits that can be employed in a given area is reduced.
This problem is avoided where the circuit is printed onto a substrate, the board is then applied and the board and circuit are stripped from the substrate. However, as has been noted above, such an arrangement tends to put strains on the circuit and may cause it to become cracked or weakened with a further increase in resistivity. It is, therefore, a still further object of this invention to provide an improved method for forming a printed circuit on a board wherein a wide density of circuits can be formed in a given cross-sectional area and the circuits will have low resistivity and be free of cracks.
One application of printed circuits which has recently obtained some interest is the use of such circuits or portions of them as a thermocouple. As is well known, a thermocouple is an electrical device for measuring temperature wherein a pair of dissimilar electrical conductors are connected to each other at a junction. The dissimilar metal junction causes an electrical signal to be given that is related to the temperature of the junction and hence this device can be used for providing temperature indication. It has been proposed to use a printed type circuit for such a thermocouple wherein two dissimilar metals are plated onto an insulating base and in overlying relationship to form the thermocoupled junction. Such devices have wide utility and permit the measuring of temperature variants over a very small surface area or the average temperature of a small area. With such thermocouples, it is extremely important that the junction not be upset and that it be strong and capable of withstanding shock.
It is a further object of this invention to apply the aforenoted methods of forming printed circuit boards of the film type to the formation of a thermocouple.